The FBI announced that it has concluded its investigation into the 2001 anthrax mailings, saying Friday that a biodefense researcher carried out the attacks alone.

The anthrax letters killed five people and sickened 17 shortly after the September 11, 2001, terrorist attacks. The letters, filled with bacterial spores, were sent to Senate Democratic leaders and news organizations.

The investigation into the anthrax mailings, code-named "Amerithrax," was one of the largest and most complex in the history of law enforcement, according to the FBI.